Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. Such recording materials become photothermographic upon incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
In U.S. Pat. No. 2,910,377 the following statement is made in the description in column 7, lines 23-27: "Stability towards exposure to light is improved by selecting highly purified materials; freedom from halides and sulphides is particularly important in the case of compositions involving silver salts". The disclosure in U.S. Pat. No. 2,910,377 concerned thermographic recording materials coated from solvent media.
WO 94/16361 discloses a multilayer heat-sensitive material which comprises: a colour-forming layer comprising: a colour-forming amount of finely divided, solid colourless noble metal or iron salt of an organic acid distributed in a carrier composition; a colour developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a colour-forming reaction with the noble metal or iron salt; and an image-toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the colour-forming layer. WO 94/16361 concerns thermographic materials coated from aqueous media.
Ever tighter solvent emission regulations and measures to avoid solvent explosions, make the avoidance of solvent coating desirable. However, thermographic materials of the type disclosed in WO 94/16361 while being coatable from aqueous media exhibit an inadequate archivability for many applications. Furthermore, the presence of chloride ions in the ingredients has been found to cause poor light stability. There is therefore a need for thermographic recording materials coatable from aqueous media based on substantially light-insensitive organic silver salts with improved shelf-life and stability to light, whose prints exhibit improved archivability and stability to incident light.